A separate function type (laminate type) electrophotographic photoreceptor having a laminate photosensitive layer composed of a charge generating layer and a charge transporting layer for performing the respective functions have undergone many improvements in charge retention, optical response, spectral characteristics, mechanical strength, and the like, and various proposals have been made with respect to the respective layers.
A charge transporting layer generally comprises a charge transporting material either alone or in combination with a film-forming binder resin. A number of charge transporting materials have been proposed to date, including pyrazoline compounds, hydrazone compounds, benzidine compounds, and polyvinylcarbazole compounds.
The separate function type electrophotographic photoreceptor is known to involve the following disadvantages upon repeated use: (1) They are susceptible to influences of corona charge products, such as ozone oxidation products, during charging processing, copying processing, etc. to cause an image blur. (2) Reductions in electrophotographic characteristics, such as an increase in residual potential and an increase in exposure potential (highlight potential), occur. It has therefore been demanded to overcome these problems.
In order to solve the above problems, an electrophotographic photoreceptor using two charge transporting materials, in which one has a higher ionization potential than that of the charge generating material and the other has a lower ionization potential than that of the charge transporting material, as in JP-A-2-293853. (The tern "JP-A" as used herein means an unexamined published Japanese patent application.)